Agriculture: Floriculture and Ornamental Nurseries Pest Management Guidelines

Downy Mildew

  • Downy mildew: Bremia sp., Peronospora spp., Plasmopara spp.
  • Symptoms and Signs

    The name downy mildew is somewhat descriptive of the fluffy appearance of the white, lavender, or purple sporulation that occurs usually on the undersides of diseased leaves. Downy mildew leaf lesions are often angular in shape and delimited by veins. Pale yellow, purple, or necrotic areas often are visible from the upper side of the leaf. Extensive disease can result in the death of large portions of the leaves.

    In some plants, when young shoots are infected, the fungus may become systemic and the resultant growth is stunted, malformed, and discolored.

    Comments on the Disease

    In contrast to powdery mildews, the downy mildews require very wet or humid conditions to flourish. Water is required for infection, and humidity above 90% is needed for sporulation. Growth of the pathogens is favored by cool temperatures.

    Downy mildew spores are usually short-lived, although they may survive several days under cool, moist conditions. They are airborne, and when they land on a susceptible plant with free water present, germination and infection generally occur within 8 to 12 hours. Some downy mildews also produce a sexual spore (oospore) that can survive dry conditions. This enables the pathogen to survive in the absence of a host. Downy mildews are favored by moist and cool conditions (40° to 60°F).

    Management

    Use varieties resistant to downy mildew. Avoid sprinkler irrigation and reduce relative humidity in greenhouses. Fungicide treatment of susceptible varieties is needed when the disease occurs on transplants or early in crop development in the field; repeated applications may be required to protect new growth. Treatment during early flowering is required on some seed crops.

    Organically Acceptable Methods

    Resistant varieties and some copper sprays are suitable for organically grown crops.

    Common name Amount to use REI‡
    (Example trade name) (hours)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. AMETOCTRADIN/DIMETHOMORPH
    (Orvego) 11–14 fl oz/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Mitochondrial respiration inhibitor (45) and cell wall synthesis inhibitor (40)
     
    B. BOSCALID/PYRACLOSTROBIN
      (Pageant) 12–18 oz/100 gal 12
      MODE OF ACTION GROUP NAME (NUMBER1): Carboximide (7) and quinone outside inhibitor (11)
     
    C. FIXED COPPER
    (Kocide 2000)# Label rates 48
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 01)
    COMMENTS: A protectant fungicide. Growth of some plants may be reduced by this material; follow label directions carefully to reduce the risk of phytotoxicity. Not all copper compounds are approved for use in organic production; be sure to check individual products.
     
    D. CYAZOFAMID
      (Segway) 2.1–3.5 fl oz/100 gal water 12
      MODE OF ACTION GROUP NAME (NUMBER1): Ubiquinone reductase, Qi site (21)
      COMMENTS: Toxic to aquatic organisms.
     
    E. FENAMIDONE
      (Fenstop) 7–14 fl oz/100 gal water 12
      MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
      COMMENTS: Toxic to aquatic organisms.
     
    F. FLUOPICOLIDE
      (Adorn) 1–4 fl oz/100 gal water 12
      MODE OF ACTION GROUP NAME (NUMBER1): Benzamides (43)
      COMMENTS: Toxic to aquatic organisms.
     
    G. SALTS OR ESTERS OF PHOSPHOROUS ACID
    (Aliette WDG) 2.5–5 lb/100 gal water 12
    MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (P 07)
    COMMENTS: For control of downy mildew on roses. Spray to wet using no more than 400 gal/acre.
     
    H. MANCOZEB
    (Dithane 75DF) 1–1.5 lb/100 gal water 24
    MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M 03)
    COMMENTS: A protectant fungicide. Thorough coverage is important for control.
     
    I. MANDIPROPAMID
      (Micora) 4–8 fl oz/100 gal 4
      MODE OF ACTION GROUP NAME (NUMBER1): Carboxylic acid amides (40)
     
    J. MEFENOXAM
    (Subdue Maxx) 0.5–1.0 fl oz/100 gal water 48
    MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)
    COMMENTS: Tank-mix with a non-Group 4 fungicide labeled for downy mildew.
     
    K. REYNOUTRIA SACHALINENSIS#
      (Regalia CG) Label rates 4
      MODE OF ACTION GROUP NAME (NUMBER1): anthraquinone elicitor (P 05)
     
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a different mode of action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action group number.
    # Acceptable for use on organically grown ornamentals.
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.
    Text Updated: 11/20
    Treatment Table Updated: 11/20